This invention relates to a method of compacting freeze-vacuum-dehydrated particulate foods, such as vegetables and meats, so that the dehydrated foods will have substantially greater bulk densities than they do when dehydrated to low moisture contents without compaction, and so that upon rehydration the foods will be restored to substantially the same distinct particulate states in which they existed prior to compaction.
Freeze drying of foods is a well-known method for preserving foods so that they can be stored at room temperature or even higher temperatures for long periods of time, provided they are maintained at very low moisture levels during such storage. When properly prepared and stored, such foods may be rehydrated at the time it is desired to consume them, the reconstituted foods being of quality approaching that of freshly prepared foods of the same types.
In recent years, the military forces have found it to be highly desirable to compact freeze-dried foods so that the foods will occupy appreciably less space in submarines, in aircraft, and also when carried on the person, which may be necessary at times when soldiers are operating in the field.
One of the more widely accepted of conventional methods of compacting freeze-dried foods is in accordance with Ishler et al, U.S. Pat. No. 3,385,715, whereby a food is first freeze-dried to a low moisture level, such as about 1.1 to about 2.2%. Then the freeze-dried food is sprayed with enough water to raise its average moisture content to from about 5 to about 13% so that, when compaction of the freeze-dried food is carried out, the food will not shatter, but will flow while maintaining its cellular structure and the particles of food will adhere to each other when compacted, the moisture serving as a plasticizer and minimizing shattering of the food into fine powder. However, in order for adequate plasticization of the freeze-dried food to occur, the food particles are sprayed with water or an aqueous solution of a gum or the like and are then permitted to stand long enough for the water to substantially equilibrate throughout the food particles. This equilibration process may take several hours (or even days at low temperatures) to occur sufficiently to produce high quality compacted freeze-dried foods. After compaction is carried out, the compacted food is usually freeze-vacuum-dehydrated to a moisture level sufficiently low for the retention of good quality after storage of the type planned for the food. This, of course, means that a substantial amount of freeze drying, an expensive process in itself, has to be repeated to remove the water with which the food particles are sprayed and equilibrated to plasticize the food during compaction. Furthermore, the slowness of the equilibration makes it virtually necessary for the dehydration and compaction process to be carried out in stages rather than in a continuous manner, as would be desirable.
Attempts have been made to avoid the above-mentioned repetition of freeze-drying with the attendant high cost thereof by stopping the initial freeze-drying of the food particles at a moisture level of from about 5 to about 13% and then compacting the partially freeze-dried food. However, as is well known in the food freeze drying art, the cores of the food particles being freeze vacuum dehydrated remains frozen until most of the moisture that is removed from the food has sublimed and the water vapor formed thereby has migrated through the outer portions of the food particles under the influence of the vacuum. Thus, if the freeze-vacuum-dehydration is stopped at 5 to 13 percent moisture content (overall average), as disclosed in the Ishler et al patent, the outer portions of the particles of food are extremely dry while the cores contain essentially their original concentrations of water as ice. If such food particles are then compacted without permitting equilibration to occur, the extremely dry outer portions of the particles shatter and the compacted product has very poor properties, such as poor adhesion and consequent ease of shattering, even with careful handling, and inability to restore the original particle sizes and shapes upon rehydration. To avoid such results, the particles of partially freeze-dried food have been held in closed containers at temperatures above 0.degree. C. to permit equilibration of the moisture from the frozen cores throughout the particles of food. Such equilibration, however, proceeds very slowly; and the larger the particles, the more slowly equilibration occurs. In some cases, a matter of days of equilibration may be required before compaction of the food may be carried out without producing excessive amounts of fines by the shattering of the dry outer portions of the food particles.
Freeze-vacuum-dehydration is inherently a slow and expansive process; and the more repetition of the freeze drying that is necessitated in connection with the compaction of freeze-dried foods, the more expensive the ultimate product will be.
It is an object of the present invention to provide an improved method of compacting freeze-dried particulate foods to relatively high bulk densities so that upon rehydration the foods will be restored to substantially the same distinct particulate form in which they existed prior to compaction of the foods and so that the reconstituted foods will be of high quality and as closely comparable as possible to the original raw or cooked foods from which the freeze-dried particulate foods are prepared.
Other objects and advantages will become apparent from the following description of the invention.